1. Field of Invention
The invention relates to an EUV source (EUV=extreme ultraviolet) in which EUV radiation is produced by a high temperature plasma which has been produced by a discharge, such as, for example, an EUV source which is used for a semiconductor lithography device, bioanalysis, material structural analysis, or the like.
2. Description of Related Art
An EUV source of the so-called Z-pinch type as is described, for example, in Japanese patent disclosure document JP-A-2002-507832 (and corresponding U.S. Pat. No. 6,075,838), is known as a light source which is used for semiconductor lithography or the like and in which EUV radiation with a wavelength from roughly 10 nm to 15 nm is produced. Here, the following takes place:                an emission gas such as xenon gas or the like is introduced into the space between the anode and the cathode;        afterwards, an electrical pulse with high energy is applied between the anode and the cathode and a discharge current is allowed to flow;        the current is allowed to be pinched by its own magnetic field which is formed hereby, in the direction to its center axis; and        as a result, plasma with a high temperature and a high density is produced, and thus, EUV radiation is generated.        
Japanese patent disclosure document JP-A-2003-518316 (and corresponding U.S. Pat. No. 6,188,076) shows a process with a so-called capillary tube discharge, in which the following is carried out:                a cathode and an anode are placed on the two ends of an insulator constituted by a narrow tube with a narrow opening;        a pulse voltage is applied between the electrodes;        by closing the discharge current which is flowing, the current density is increased by the wall of the narrow tube;        as a result, a high temperature plasma is produced and EUV radiation is allowed to form.        
In each of the above described EUV sources, EUV radiation is emitted by a high temperature plasma which is produced by the discharge. The EUV radiation which has been formed emerges to the outside from the discharge part, is routed, for example, to an exposure device for semiconductor lithography, and is used.
The EUV radiation is easily absorbed by the material. When there is residual gas or the like in the path of the radiation, it is absorbed by it, by which its intensity is reduced. If, for example, EUV radiation with a wavelength of 13 nm propagates 1 m in xenon gas with a pressure of 10 Pa, its intensity decreases to roughly 1/500. The attenuation factor of EUV radiation differs depending on the type of residual gas. However, it is necessary to evacuate such that the pressure of the residual gas in the area which corresponds to the path of the EUV radiation is as low as possible, for example, at most 1 Pa.
In the prior art, within a hermetically closed vessel, there is a discharge part. The discharge gas is supplied from one side of the space between the cathode and the anode (discharge space). The discharge gas is allowed to escape from the other side. The discharge gas which has been allowed to escape from the discharge space to the outside is evacuated by a pump from the hermetically closed vessel in order to suppress as much as possible the attenuation of the EUV radiation by the residual gas.
FIG. 4 shows one example of the arrangement of the discharge part according to the prior art.
In FIG. 4, a first electrode 11 (anode), a second electrode 12 (cathode), and a discharge tube 13 are shown. The discharge tube 13 is clamped as an insulator between the first electrode 11 and the second electrode. The first electrode 11 and the second electrode 12 are connected to a pulse current source from which a heavy current pulse is supplied. The discharge gas 25 is introduced through an opening of one electrode, e.g., cathode 12 into the discharge tube (insulator) 13 and is allowed to escape through the opening of the other electrode, e.g., anode 11. Here, the distribution of the pressure of the discharge gas which has been introduced into the discharge space before starting the discharge (initial gas pressure) in the direction of the optical axis from curve C1 is shown in the graph at the bottom in FIG. 4. It can be imagined that it is high on the side of gas supply and is low on the side of gas escape. As was described above, the loss by absorption is smaller, the lower the residual gas pressure in the area in which the EUV radiation is propagating. Normally, the EUV radiation is therefore allowed to escape on the gas escape side and used.
If, in the arrangement of the discharge part shown in FIG. 4, the gradient of the initial gas pressure in the discharge space is large in the direction of the optical axis, the problem arises that the efficiency of the conversion of input electrical energy into EUV radiation energy in the desired wavelength range (hereinafter, also called only conversion efficiency) decreases. Even if the electrical energy consumed for discharge is the same, the area of the easily emittable wavelength differs when the temperature and the density of the generated plasma differ.
In order to obtain EUV radiation with the desired wavelength with high efficiency, it is therefore necessary for the temperature and the density of the plasma to be in a suitable parameter range. The wider the area in which plasma is produced within this parameter range, the greater the light intensity in the required wavelength range of the EUV radiation obtained and the higher the conversion efficiency becomes.
However, if the initial gas pressure has a gradient and if the initial gas density is nonuniform in space, the temperature and the density of the plasma which has been heated by the discharge become nonuniform in space and the area of the plasma which has an optimum parameter range becomes narrow. As a result, the conversion efficiency is reduced.
When the gradient of the initial gas pressure is reduced, the uniformity of the plasma increases. In order to reduce the gradient of the initial gas pressure in the conventional arrangement of the discharge part, the flow quantity of the supplied gas and the pressure on the gas supply side must be reduced.
The reason for this is the following:
As described above, to prevent loss of EUV radiation by the residual gas, it is necessary to substantially expose the gas escape side to vacuum evacuation. The gradient of the initial pressure cannot be reduced by increasing the pressure on the gas evacuation side.
If the pressure on the gas supply side is reduced, the distribution of the initial gas pressure in the direction of the optical axis is plotted by the curve C2 in the graph in FIG. 4, bottom. The gradient decreases. However, since the pressure value also decreases overall, the absolute density of the plasma which has been produced by the discharge decreases. Here, the disadvantage arises that EUV radiation emergence with the required magnitude cannot be achieved.
As was described above, in the arrangement of the discharge part in the prior art, it is difficult to achieve both an increase in conversion efficiency and also an increase of light intensity at the same time.